Thin Lens Optics

Physics 11

Thin Lens Optics

If we have a lens that has a small diameter when compared to the focal length, we can use geometrical optics to investigate the behaviour of a lens in much the same way we addressed mirrors

Two Types

We will consider two types of lens: Double concave Double convex

However, there are many other types of lenses that can be considered

Double Convex

Double Convex

Use geometrical optics for a lens with a focal length of 3cm and an object of 1cm in height placed: At 5cm At 3cm At 1cm

In each case, measure the height of the image, the placement of the image and whether the image is real or virtual, upright or inverted

Double Concave

Double Concave

Use geometrical optics for a lens with a focal length of 4cm and an object of 1cm in height placed: At 6cm At 4cm At 2cm

In each case, measure the height of the image, the placement of the image and whether the image is real or virtual, upright or inverted

Conventions

Distance (d, s) is positive for real objects/images and negative for virtual objects/images

Focal length (f) is positive for converging mirrors/lenses and negative for diverging mirrors/lenses

Height (h) is positive for objects/images that point up and negative for objects/images that point down

Equations

o

i

o

i

io

h

h

d

dm

ddf

111

Problems

Use geometrical optics to determine the position and nature of an image if: Convex mirror, radius of curvature 5cm, object

height 2cm placed 4cm from the mirror Concave mirror, radius of curvature 3cm,

object height 0.5cm placed 1cm from the mirror

Double concave lens, focal length 10cm, object height 1cm placed 3cm from the lens

Double convex lens, focal length 1cm, object height 3cm placed 0.5cm from the lens